1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission and more particularly to a shift device for controlling a shift between gear ratios in an automatic transmission.
2. Description of the Prior Art
The automatic transmission is designed such that gear ratios are selectively established by switching the friction element or elements, such as a clutch or a brake, to be engaged, and the control of a shift is carried out by shift valves provided in a hydraulic control system. (For example, in the case of a forward three speed automatic transmission, the shift control is carried out by two shift valves, vis., a 1-2 shift valve and a 2-3 shift valve.) Commonly, a conventional shift valve is constructed such that a throttle pressure indicative of an opening degree of a throttle valve of an engine and a governor pressure indicative of a vehicle speed of a motor vehicle are exerted on a spool of the shift valve in opposed directions, allowing the spool to shift between two positions in response to the relationship in magnitude between them, thus supplying or discharging a line pressure to or from a corresponding friction element or elements. Therefore, a shift line, vis., a throttle opening degree vs., vehicle speed characteristic, on which the shift valve switches from one state to another state has been a straight line or a quadratic curve. With this shift line, it is difficult to provide a drive feel fit for the characteristic of an engine and that of a motor vehicle over the whole throttle opening degrees. For example, if shift points are set fit for operation within a relatively large throttle opening degree range, shift points become too high to be appropriate or too low to be appropriate for operation within a medium or small throttle opening degree range, thus failing to provide appropriate shift points. A conventional shift valve is provided with an oil pressure acting area on which an oil pressure (throttle pressure or line pressure or the like) acts when a spool is in a downshift position (for example, in the case of a 2-3 shift valve, the term "downshift position" is herein used to mean a position which the spool of the shift valve takes in a second gear ratio state and the term an "upshift position" is used to mean a position which the spool takes in a third gear ratio state) so as to provide a hysteresis between a shift point for an upshift and a shift point for a downshift but, with this construction, it is difficult to provide a desired hysteresis at any given throttle opening degree over the whole throttle opening range and thus a shift point for an upshift may not be set in a desired manner or in an independent manner from the setting of a shift point for a downshift. In summary, since, in the case of the conventional shift valve, throttle pressure and governor pressure are compared with each other by a single shift valve to determine a shift point, it is a problem that since the shift points can not be chosen freely, a shift point for an upshift and a shift point for a downshift can not be set in a desired manner over the whole throttle opening degree range.